Now that Tasker can answer calls as of v1.0.8, I set up an auto-answer for when my bluetooth headset is connected.
Note: I had found that my original setup had the annoying side effect of automatically picking up a call that came in when I was already on a call. Now I have improved it to NOT automatically answer if the phone is "off the hook."

May not work with all Bluetooth devices. Failed with 2 different BT devices (Jawbone and Microsoft Sync) In both cases, Even though the BT device is connected, "Take Call' answers the incoming call on the cellular device, not handsfree via the BT device. For both, to answer an incoming call via the BT device takes human interaction - whether to press the button on the Jawbone, or pressing the dashboard button for MS Sync to answer.